Some potatoe varieties are 'floury'. Maybe try a different variety next year. I grew Desiree last year and they were fine, but I also like a floury potato :o)

When you say mushy do you mean they disintegrate before they are cooked through. Do you have the same problem with shop bought potatoes? If not you could buy seed potatoes of the variety you would usually buy in the shops and see if that makes any difference.

This was discussed just recently, It seems that lack of water during the growing period means that the potato acts like a sponge when boiled, absorbing water too rapidly.

Grow the 'salad potato' varieties like Nicola and Belle de Fontenay. Alternatively, a good large potato for main crop is Mona Lisa.

My Belle de Fontenay were lovely,and didn't turn to mush at all when cooked.Neither did Vales Emerald..so I think the variety is the key..also had Nicola the year before,and Swift,which were also good ones..I have never grown maincrop,as I haven't the space..